A Tyrosinase-Responsive Nonenzymatic Redox Cycling for Amplified Electrochemical Immunosensing of Protein.

Electrochemical immunoassays with high sensitivity and a wide dynamic range are still a challenge in clinical diagnosis. This study reports a novel tyrosinase (Tyr)-responsive nonenzymatic redox cycling for significantly amplifying the electrochemical signal produced from Tyr-labeled immunoassays. The tyrosinase could be captured on the immunosensor surface by a sandwich immunoreaction. The immunosensor was conveniently prepared by covalently binding capture antibody to a chitosan-coated electrode. Using phenol as a substrate and NADH as reducing agent, which showed negligible background due to low electroactivity of phenol and high oxidation overpotential of NADH, the oxygenation activity of tyrosinase could convert poorly electroactive phenol to highly electroactive catechol to trigger an NADH-related nonenzymatic electrochemical-chemical (EC) catalysis. The EC redox cycling leads to a high signal-to-noise ratio for immunoassays. Using carcinoembryonic antigen (CEA) as an analyte model, the amplified immunoassay showed excellent performance with a detectable range from 1.0 pg/mL to 0.1 μg/mL and a sub-pg/mL-level detection limit. The acceptable accuracy and good reproducibility of the proposed immunoassay method indicated its superior suitability in clinical diagnosis.